jeannette wrote:
Possibly a check valve in front of the macerator so it isn't
constantly loaded. Mine has one. Close the valve then turn the
macerator off.

Not a check valve...a y-valve (aka diverter valve) that allows you to
choose between pumping out or going to the macerator.

However, whether the boat should even HAVE a macerator or any other pump
to dump the tank depends on whether the boat is in coastal waters that
provide immediate access to open sea...'cuz it's illegal to dump a tank
in ANY US waters....only when at least 3 miles out to sea. And that
doesn't mean 3 miles from shore in a lake, river, bay or sound...it
means 3 miles from the nearest point on the whole North American
coastline. So unless he has immediate access to open sea, he might as
well just remove the macerator altogether...'cuz the fines for dumping a
tank illegally are very steep.